This invention relates to heat transfer promoters and methods for using the same, and more particularly to heat transfer promoters for improving the heat transfer performances of heat transfer tubes used in vapor-liquid condensation systems and a method of using the same.
In recent years, a variety of studies have been carried out in order to improve the performances of heat exchangers from the standpoint of saving energy and saving resources.
In general, in a heat exchanger of vapor-liquid condensation systems such as a steam condenser of a steam turbine plant and a condenser of a refrigerator, the condensation performance depends largely upon the heat transfer performance of the heat transfer tubes used in the heat exchanger. Therefore the improvement of heat transfer performance is extremely important. For example, if heat transfer performance of the heat transfer tubes used in a surface steam condenser of a steam turbine plant is inferior, the condensation performance and degree of vacuum of the condenser will be reduced, whereby the thermal efficiency of the whole plant will be reduced.
In general, in order to improve the cooling performance of the condenser, a method wherein the heat transfer area of the heat transfer tubes is increased and another method wherein the flow state of cooling water fed to the condenser is increased have been proposed. However, the use of heat transfer tubes having a long length or the use of heat transfer tubes provided with fins which increase the heat transfer area leads to complication of the structure and increase of cost.
Further, if the flow rate of the cooling water circulated through the condenser is increased, the pressure loss of the cooling water significantly increases (the pressure loss is proportional to square of the flow rate). Therefore not only does the power expenditure of the pump increase, but also the corrosion rate of the heat transfer tubes increases with inincreasing flow rate.
Other effective methods for improving the heat transfer performance of the heat transfer tubes include a method wherein the condensation at the surface of the heat transfer tube is caused to be a dropwise condensation.
In general, when steam is brought into contact with a heating surface having a lower temperature than saturation temperature of the steam, the condensation mechanism is a filmwise condensation or a dropwise condensation. In the filmwise condensation, the condensate covers the surface of each heat transfer tube in the form of a film and flows downward, and the released condensation latent heat is transmitted to the heat transfer tube through the condensate film. In the dropwise condensation, steam condenses at the surface of each heat transfer tube in the form of drops and flows downward.
It has been known that the dropwise condensation exhibits extremely good heat transfer characteristics because there is no heat transfer resistance due to the condensate film formed on the surface of the heat transfer tube in the case of the filmwise condensation. For example, the coefficient of overall heat transmission during dropwise condensation is far greater than that during filmwise condensation by 10 times or more.
At the surface of each metal heat transfer tube, filmwise condensation usually occurs. Therefore, the conversion of this to dropwise condensation is extremely effective for improving heat transfer performance and is a fascinating subject. Heretofore, many studies on promoters for promoting dropwise condensation have been carried out. However, promoters capable of being applied to actual condensers have not yet been developed.
For example, promoters for copper or copper alloy cooling tubes include octyl thiocyanate, benzyl mercaptan, and the like. It has been known that dropwise condensation occurs by treating the surface of each tube with these materials. However, the bonding power between these materials and metal is weak, and therefore the materials readily peel off from the surface of the tube.
Further, dropwise condensation can be promoted by coating the surface of each heat transfer tube with film of materials having good water repellency such as tetrafluoroethylene resin (Teflon). However, in this case, heat transfer resistance is increased by virtue of the Teflon film.